1. Field of the Invention
The present invention relates generally to a process for forming a copper coating by chemical or electroless plating, and more particularly a chemical plating process suitable for forming a copper coating having excellent mechanical properties, and to a printed-wiring board with conductive wiring patterns formed of such a copper coating.
2. Discussion of the Prior Art
A copper coating is formed on a substrate by a chemical plating process wherein the substrate is immersed in a plating bath containing copper salts, reducing agents and buffer agents, and a copper material in the bath is deposited on the surfaces of the substrate due to concurrent or parallel oxidazation and reduction. The chemical plating process is useful for forming copper coatings on various electrically insulating materials, and is therefore widely practiced for various industrial applications. However, this process is disadvantageous in that the copper coatings obtained are not completely satisfactory in mechanical properties such as tensile strength and elongation.
For example, the chemical plating technique is widely utilized to form conductive wiring patterns of copper on an electrically insulating substrate, for fabricating printed-wiring boards. However, it is recognized that the copper coatings thus formed on the substrate tend to suffer from cracking due to a difference in thermal expansion coefficient between the substrate and the coating layer, and other factors. Thus, it is difficult to produce highly reliable printed-wiring boards by solely relying on the chemical plating technique. Cracking of the copper coating on the printed-wiring boards is serious, particularly where the wiring patterns of copper provided on the opposite major surfaces of the substrate are electrically connected to each other by a copper layer formed on the inner surfaces of through-holes formed through the substrate. More specifically, the copper coating is subject to heavy cracking near the openings of the through-holes, which may easily cause electrical discontinuity of the wiring patterns.
In light of the above drawbacks, it has been a conventional practice to first form a copper coating with a thickness of 0.5 to several microns by chemical plating, and then perform an electroplating process wherein the formed copper coating is connected as a cathode, and a copper material in an electrolyte solution is electrodeposited on the previously formed copper coating, whereby the eventually obtained coating is improved in its mechanical properties.
The conventional practice indicated above requires suitable electroplating equipment, and a complicated procedure to obtain a desired copper coating, which inevitably leads to increased cost of manufacture. In this connection, it is noted that one of the important advantages of a chemical plating process lies in the absence of any electrical equipment. If it is possible to form a mechanically excellent copper coating solely by the chemical or electroless plating process, the conventional need of an additional electroplating step and the electrical equipment therefor can be eliminated, and the economical advantage of the chemical plating can be enjoyed. Further, improvements in the mechanical properties of such copper coatings are desirable even in the fields of technology in which the copper coatings produced solely by the conventional electroless plating technique are almost satisfactory.